The method and system disclosed herein relates to capturing carbon dioxide (CO2) from combustion sources such as flue gas of a power plant and making the CO2 available for sequestration or other uses.
Emissions of greenhouse gases such as CO2, if left unchecked, may potentially affect climatic conditions. Conversion of fossil fuels such as coal and natural gas to energy is a source of greenhouse gas emissions. Emissions of the greenhouse gases can be reduced by various means such as increase in efficiency of the combustion process and use of renewable energy such as wind and solar but the reduction in the emission of the greenhouse gases required to stabilize the greenhouse gas levels cannot be achieved without capturing a substantial part of the greenhouse gases at the source of the greenhouse gas emissions during either the pre-combustion process or the post-combustion process. Post-combustion capture of CO2 from the flue gas of a power plant or other streams such as the flue gas from a refinery involves use of a solvent, typically an amine, which is regenerated using a part of the steam generated during the combustion process. Pre-combustion capture of CO2 involves chemical reaction of the fuel with air or oxygen and then with steam to produce a mixture of carbon dioxide and hydrogen. The carbon dioxide is removed from this stream through a CO2 capture process and hydrogen may be used as a fuel for power generation. If oxygen is used for combustion, a flue gas containing mainly carbon dioxide is produced which can be easily separated for sequestration.
The post-combustion capture of CO2 results in 9%-11% reduction in absolute efficiency for power generation and about 28-30% reduction in relative efficiency for a pulverized coal power plant as discussed by Ciferno (Ciferno, J., “A Feasibility Study of Carbon Dioxide Capture from an Existing Coal-Fired Power Plant,” paper presented at the Sixth Annual Conference on Carbon Capture and Sequestration, Pittsburgh, Pa., May 2007.). A May 2007 NETL report (Carbon Sequestration Technology Roadmap and Program Plan—2007, U.S. DOE National Energy Technology Laboratory (NETL), May 2007.) shows a 60-100% increase in cost of power generation for existing power plants taking into account capital and operating costs for CO2 separation and sequestration. Net power output from the power plant is also decreased by 30% or more. Means to significantly decrease the power and capital penalty associated with the post-combustion CO2 capture are sought. For the post-combustion capture, the Department Of Energy (DOE) has a goal of less than 35% increase in power cost for 90% CO2 capture.
Most studies dealing with the post-combustion CO2 capture use amine or ammonia-based absorption processes for the removal of carbon dioxide from the flue gas. The absorption-based processes have drawbacks such as the significant capital requirements. The best amine based absorbents such as the hindered amines and amine blends have an energy requirement in the range of 750-900 Kcal/kg (1,350-1,620 Btu/lb) of the CO2 captured. Furthermore, amine-based processes require the use of specialty steel equipment and the associated capital investment because of the corrosive nature of amine and ammonia solutions in the presence of acidic gases and oxygen. This equipment represents a significant capital cost.
In contrast to the amine-based systems, the heats of adsorption of CO2 on various zeolite and carbon based adsorbents range between 140-240 kcal/kg or 252-432 Btu/lb (Valenzuela, D. P. and A. L Myers, “Adsorption Equilibrium Data Handbook,” Prentice Hall, Englewood Cliffs, N.J., 1989.) which is about a fifth of the heat of absorption for the amine-based systems. There is an unmet need for practical adsorption systems that can take advantage of low heats of adsorption while providing high carbon dioxide yield and high recovery.
Temperature swing adsorption systems have been used extensively for applications such as air drying, natural gas drying, and water and CO2 removal prior to cryogenic distillation of air. These systems typically remove less than 2% of impurities and the regeneration outlet stream containing the impurities is not of high purity. Also the typical temperature swing adsorption processes have adsorption times of the order of 4-12 hours. For feed CO2 concentrations between 10-12% in the flue gas, these adsorption times would require extremely large adsorption beds. For example, assuming a working capacity of 12 weight % (difference in capacity between the adsorption and the regeneration steps), an adsorbent density of about 660 kgs/m3 and an adsorption time of 4 hours, a plant processing 1000 tons/day of CO2 in the feed would require about 8,000 m3 (5.3 million kilograms) of the adsorbent, a size that makes the systems not practical for capturing carbon dioxide from combustion sources.
The method and system disclosed herein provides a solution for the efficient capture of CO2 using a process based on a temperature and pressure swing adsorption cycles.